Photodegradation Performance of g-C3N4 Fabricated by Directly

Jul 16, 2009 - E-mail: [email protected] (Z. S. Li). Postal address: NO. 22, Hankou Road, Nanjing, Jiangsu 210093, P. R. China. Phone number: 86-25-8368...
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Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine S. C. Yan,†,§ Z. S. Li,*,†,‡,§ and Z. G. Zou†,‡,§ †

Eco-Materials and Renewable Energy Research Center (ERERC), Department of Physics and ‡Department of Materials Science and Engineering and §National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China Received March 17, 2009. Revised Manuscript Received July 3, 2009

The g-C3N4 photocatalyst was synthesized by directly heating the low-cost melamine. The methyl orange dye (MO) was selected as a photodegrading goal to evaluate the photocatalytic activity of as-prepared g-C3N4. The comparison experiments indicate that the photocatalytic activity of g-C3N4 can be largely improved by the Ag loading. The strong acid radical ion (SO42- or NO3-) can promote the degrading rate of MO for g-C3N4 photocatalysis system. The MO degradation over the g-C3N4 is mainly attributed to the photoreduction process induced by the photogenerated electrons. Our results clearly indicate that the metal-free g-C3N4 has good performance in photodegradation of organic pollutant.

Introduction In recent years, there has been increasing interest in the study of photocatalysis technology application in water/air purification,1 hydrogen production from splitting water,2 self-cleaning coatings,3 and high-efficiency solar cells.4 This is related to serious environmental pollution and the global energy shortage. So far, various oxide, sulfide, and oxynitride semiconductor photocatalysts have been developed to drive the photocatalytic reaction.5 However, one of the main goals in materials science fields is to find photocatalytic materials with high quantum efficiency. For water purification, an optical material is needed that has a band gap that absorbs visible light, strong oxidative ability, and high stability in a complex water solution system. Recently, Wang et al.6 reported that a metal-free photocatalyst, graphite-like carbon nitride (g-C3N4), has the photocatalytic performance for hydrogen or oxygen production from water splitting under visible light irradiation. Very recently, they developed the g-C3N4 metalincluding compounds to degrade organic dyes.7 The functional organic-metal hybrid material exhibited modified electronic properties. However, the authors did not report the comparison results for degrading organic dyes over the bare and metal-ionmodified g-C3N4. The g-C3N4 semiconductor is recognized to be the most stable allotrope at ambient conditions. Unlike the photocatalysts of sulfide and oxynitride semiconductor, the g-C3N4 photocatalyst is stable under light irradiation in water solution as well as in acid (HCl, pH=0) or base (NaOH, pH=14) solutions due to the strong covalent bonds between carbon and *Corresponding author. E-mail: [email protected] (Z. S. Li). Postal address: NO. 22, Hankou Road, Nanjing, Jiangsu 210093, P. R. China. Phone number: 86-25-83686630. Fax number: 86-25-83686632. (1) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000 1, 1–21. (2) Yamasita, D.; Takata, T.; Hara, M.; Kondo, J. N.; Domen, K. Solid State Ionics 2004, 172, 591–595. (3) Zhang, X. T.; Sato, O.; Taguchi, M.; Einaga, Y.; Murakami, T.; Fujishima, A. Chem. Mater. 2005, 17, 696–700. (4) O0 Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–740. (5) Osterloh, F. E. Chem. Mater. 2008, 20(1), 35–54. (6) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson J. M.; Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76–80. (7) Wang, X. C.; Chen, X. F.; Thomas, A.; Fu, X. Z.; Antonietti, M. Adv. Mater. 2009, 21, 1–4.

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nitride atoms. The high stability and moderate band gap imply that the metal-free g-C3N4 has numerous potential applications in the photocatalysis fields. Several precursors, such as cyanamide, dicyandiamide, and melamine, have been used to obtain the g-C3N4 solids.8 The former two kinds of precursors are virulent and expensive in comparison to the latter one. So far, several research groups have reported that the g-C3N4 can be fabricated by heat treatment of melamine in the low-vacuum system9,10 or under high pressure.11 Depending on reaction conditions, g-C3N4 with different degrees of condensation and properties was obtained. However, from an industrial applications viewpoint, the g-C3N4 material obtained under ambient pressure was expected due to the low cost and facile operation. In the present paper, the g-C3N4 photocatalyst powder was obtained by directly heating the melamine in a semiclosed system with two-step heat treatment. The melamine was first heated at 500 °C (heating rate: 20 °C /min) for 2 h, and the further deammonation treatment was set at 520, 550, and 580 °C for 2 h, respectively. For comparison, a reference sample was prepared by polymerization of cyanamide at 550 °C for 4 h.6 The photodegradation behavior of methyl orange (MO) over the as-prepared photocatalyst was studied. The sample of treated melamine at 520 °C for 2 h exhibits high photodegradation activity. Our studies probably imply that the g-C3N4 has the potential to treat industrial wastewater due to its good photodegradation performance and environmental adaptability. On the basis of our comparison experiments, a possible MO photodegradation mechanism for the g-C3N4 photocatalysis system was proposed.

Experimental Section The photocatalyst of g-C3N4 was prepared by directly heating melamine in the semiclosed system to prevent sublimation of (8) Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; M€uller, J.; Schl€ogl, R.; Carlsson, J. M. J. Mater. Chem. 2008, 18, 4893–4908. (9) Komatsu, T. J. Mater. Chem. 2001, 11, 799–801. (10) Zhao, Y. C.; Yu, D. L.; Yanagisawa, O.; Matsugi, K.; Tian, Y. J. Diamond Relat. Mater. 2005, 14, 1700–1704. (11) Ma, H. A.; Jia, X. P.; Chen, L. X.; Zhu, P. W.; Guo, W. L.; Guo, X. B.; Wang, Y. D.; Li, S. Q.; Zou, G. T.; Zhang, G.; Bex, P. J. Phys.: Condens. Matter 2002, 14, 11269–11273.

Published on Web 07/16/2009

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Figure 1. TG-DSC thermograms for heating the melamine (a) and the g-C3N4 (b) obtained by heating melamine at 520 °C. melamine. 10 g of melamine powder was put into an alumina crucible with a cover, then heated to 500 °C in a muffle furnace for 2 h at a heating rate of 20 °C/min; the further deammonation treatment was performed at 500, 520, 550, and 580 °C for 2 h, respectively. The samples were characterized by X-ray diffraction (XRD) for phase identification on the Rigaku RINT2000 diffractometer. The specific surface area was determined with the Brunauer-Emmett-Teller (BET) equation at 77 K by using an adsorption apparatus (Micromeritics TriStar, USA). UV-vis diffuse reflection spectra were measured using a UV-vis spectrophotometer (Varian CARY 100, USA) and converted from reflection to absorbance by the Kubelka-Munk method. Photocatalytic activity of g-C3N4 for methyl orange (MO) photodegradation was evaluated in a Pyrex reactor. 0.3 g of g-C3N4 was dispersed in MO aqueous solution (100 mL, 0.4 mg L-1). The light irradiation system contains a 300 W Xe lamp with cutoff filter L42 for visible light and a water filter to eliminate the temperature effect. Measurement of apparent quantum efficiency for degrading MO over g-C3N4 was performed by using a monochromatic filter (420 nm). The intensity of the corresponding incident light is 9.764 μW/cm2.

Results and Discussion In order to understand the phase transformation during heating of melamine, the thermal analysis was carried out by using the thermogravimetric-differential scanning calorimetry analysis (TG-DSC). The detected range of temperature is from room temperature to 1000 °C at a heating rate of 10 °C /min. An alumina crucible with a cover was used during thermal analysis to prevent sublimation of melamine. The DSC and TG thermograms for melamine (see Figure 1a) clearly show that in the semiclosed system several phase transformations are observed. The strongest endothermal peak appears in the temperature range 297-390 °C, and the weight of the sample decreased rapidly. It indicates that the sublimation and thermal condensation of melamine occurred in this temperature range at the same time. The inset of Figure 1a shows that two weak endothermal peaks appeared at 545 and 630 °C, which correspond to the further deammonation process and decomposition of material, respectively. Two exothermic reaction peaks are attributed to the disappearance of this material via generation of nitrogen and cyano fragments, exhibiting at 660 and 750 °C, respectively. The thermal stability and phase transformation of the g-C3N4 prepared at 520 °C were detected in an open system, as shown in Figure 1b. It can be seen that the as-prepared g-C3N4 becomes unstable when the heat temperature is above 600 °C, and heating to 750 °C results in no residue of the material being observable. The DSC curve shows that decomposition of the as-prepared g-C3N4 occurred first at 600 °C, with the decomposition products 10398 DOI: 10.1021/la900923z

Figure 2. XRD patterns for g-C3N4 samples obtained by heating different precursors at various temperatures.

burnt immediately. No deammonation process was observed during heating of g-C3N4. Compared with the DSC curve of g-C3N4, we can confirm that the endothermal peak at 630 °C during heating of melamine should be attributed to the decomposition of g-C3N4 formed in the pyrolysis of melamine, which is 30 °C higher than the starting decomposition temperature of as-prepared g-C3N4 (600 °C). This indicates that g-C3N4 is more stable in the semiclosed ammonia atmosphere than in the open system. On the other hand, two combustion processes were found during heating melamine; however, just one combustion process was observed for heating the g-C3N4. The aforementioned evidence can be attributed to the fact that the deammonation process of melamine resulted in an ammonia atmosphere in the semiclosed system, which can improve the stability of g-C3N4 and restrain combustion of products. The XRD patterns for all the samples are shown in Figure 2. We can see that two peaks are found in all the samples. It is widely accepted to date that the g-C3N4 is based on tri-s-triazine building blocks.8 The strongest peak at 27.41° is a characteristic interlayer stacking peak of aromatic systems, indexed for graphitic materials as the (0 0 2) peak. The calculated interplanar distance of aromatic units (d = 0.325 nm) is significantly smaller than that of the crystalline g-C3N4 (d = 0.34 nm).12 The dense structure can be attributed to the localization of the electrons and stronger binding (12) Bojdys, M. J.; M€uller, J. O.; Antonietti, M.; Thomas, A. Chem.;Eur. J. 2008, 14, 8177–8182.

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Figure 3. FT-IR spectra for g-C3N4 fabricated from different precursors.

between the layers. The small angle peak at 13.08°, corresponding to interplanar distance of 0.676 nm, is indexed as (1 0 0), which is associated with interlayer stacking. The distance is slightly below the size of one tris-s-triazine unit (ca. 0.73 nm), which presumably is attributed to the presence of small tilt angularity in the structure. The Fourier transform infrared (FT-IR) spectra of two kinds of samples are shown in Figure 3. Several strong bands in the 1200-1650 cm-1 region were found, which correspond to the typical stretching modes of CN heterocycles.13 Additionally, the characteristic breathing mode of the triazine units at 801 cm-1 was observed. It should be noted that the broad bands at around 3000 cm-1 are indicative of NH stretching vibration modes.12 This indicates that the amino functions still existed in the products by directly heating the melamine or cyanamide. The C/N ratios for all samples were determined by elemental analysis on the elemental analyzer (vario EL II, Elementar, Germany). The experimental error in weighing was (0.001 mg. For heating melamine at 500, 520, 550, and 580 °C, the C/N ratio of the product is 0.721, 0.735, 0.737, and 0.742, respectively. It indicates that the C/N ratios increase with increasing condensation temperature. However, the C/N ratios for all cases of heating melamine are lower than 0.75 for the ideal crystal g-C3N4. The results are in agreement with the FT-IR, implying that the amino groups originated from the incomplete condensation of the asprepared g-C3N4. Indeed, as reported, the residual hydrogen atoms bind to the edges of the graphene-like C-N sheet in the form of C-NH2 and 2C-NH bonds.14 On the other hand, as we know from DSC observation, the as-prepared g-C3N4 is unstable above 600 °C. The existence of amino groups and the low thermal stability of g-C3N4 indicate that it is very difficult to lower the hydrogen content by directly heating melamine at ambient pressure. The optical properties of the samples by heating melamine were investigated by UV-vis diffuse reflectance spectroscopy, and the results are shown in Figure 4. The spectrum of the sample of cyanamide treated at 550 °C for 4 h is also shown for comparison. We can see that absorption edges of the samples obtained from heating melamine shift remarkably to longer wavelengths with increasing heating temperature. The decrease in band gaps of the samples is from 2.8 to 2.75 eV when the heat treatment temperature is increased from 500 to 580 °C. It is worth noting that the (13) Li, X. F.; Zhang, J.; Shen, L. H.; Ma, Y. M.; Lei, W. W.; Cui, Q. L.; Zou G. T. Appl. Phys. A: Mater. Sci. Proc. 2009, 94, 387–392. (14) Zhao, Y. C.; Liu, Z.; Chu, W. G.; Song, L.; Zhang, Z. X.; Yu, D. L.; Tian, Y. J.; Xie, S. S.; Sun, L. F. Adv. Mater. 2008, 20, 1777–1781.

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Figure 4. UV-vis absorption spectra for commercial N-dopedTiO2 and g-C3N4 samples obtained by heating the different precursors at different temperatures.

main absorption of the sample heated at 520 °C, which exhibits a band gap of ca. 2.75 eV, is close to that of the samples heated at 550 and 580 °C. The spectra of the samples heated at higher temperature, 520 °C, show a weak absorption tail, especially for the sample of cyanamide heated at 550 °C, for which the absorption tail may reach 550 nm. This probably is attributed to the structure defects formed in samples treated at the high temperatures, which improve the optical absorption of materials. The photocatalytic activities of all the samples obtained by pyrolysis of melamine were evaluated for methyl orange (MO) photodegradation under visible light (λ > 420 nm) irradiation, as shown in Figure 5a. The photodegradation of MO over the commercial nitrogen-doped-TiO2 (TPS201, Sumitomo Corp. Japan) (denoted as N-TiO2) and the sample obtained by heating cyanamide was also given here for comparison. It is well-known that, as a typical organic contaminant, MO is stable under UV-vis irradiation if there is no photocatalyst involved.15,16 The absorption spectrum of the homogeneous MO solution without catalyst submitted to illumination with UV-vis light irradiation from a 300 W xenon lamp for 5 h exhibits almost no difference from the original absorption. We can know that the MO is stable in our experimental conditions. The N-TiO2 possesses a specific surface area (64.8 m2/g) more than eight times higher than that (an average value, ca. 8 m2/g) of g-C3N4 samples; however, it is obvious that photocatalytic activities for all the g-C3N4 samples are much higher than those of the N-TiO2. The optical absorption spectrum of N-TiO2 was shown in Figure 4. We can know that the main absorption edge of N-TiO2 is at 390 nm, corresponding to the essential light absorption of TiO2.17 The weak absorption tail in the wavelength range 390-480 nm originated from the substitutional doping of N, because its p states contribute to narrowing of the band gap by mixing with O 2p states.18 The weak light absorption of N-TiO2 means poor visible light response. As a result, N-TiO2 has a low activity for photodegrading MO dye. The g-C3N4 sample obtained from heating melamine at 520 °C (denoted as M520) shows the highest activity, which is the same as that of the sample obtained from (15) Wang, Y. Y.; Zhou, G. W.; Li, T. D.; Qiao, W. T.; Li, Y. J. Catal. Commun. 2009, 10, 412–415. (16) Gao, F.; Chen, X. Y.; Yin, K. B.; Dong, S.; Ren, Z. F.; Yuan, F.; Yu, T.; Zou, Z. G.; Liu, J. M. Adv. Mater. 2007, 19, 2889–2892. (17) Rajeshwar, K.; de Tacconi, N. R.; Chenthamarakshan, C. R. Chem. Mater. 2001, 13(9), 2765–2782. (18) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271.

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Figure 5. Photodegradation of methyl orange: (a) over the commercial N-doped-TiO2 and the g-C3N4 obtained by heating the different precursors at different temperatures; (b) in the different catalytic systems based on the g-C3N4 prepared by heating melamine at 520 °C.

heating cyanamide at 550 °C (denoted as C550); the dye of MO is completely degraded after 5 h visible light irradiation. However, the pathway of MO photodegradation is different for the M520 and C550 samples (see Figure S1 in the Supporting Information). No light absorption peak shift was observed during degradation of MO over M520 sample, indicating that the M520 sample can directly mineralize MO without intermediate products. For the C550 sample, the main absorption peak of MO gradually shifted to the shorter wavelengths with increasing light irradiation times, implying that the intermediate products were formed during the photodegradation reaction process. In other words, this means that the photooxidation ability of M520 is better than that of C550 sample. In the UV-visible absorption spectrum of C550, the obvious absorption tail, which resulted from poor thermal stability of g-C3N4, suggests that the high-temperature heat treatment at 550 °C leads to the decreased photooxidation ability for C550. For the samples of melamine heated at 500, 550, and 580 °C, only 89%, 78%, and 69% MO, respectively, was photodegraded in the same irradiation time. Indeed, compared with the M520 sample, the sample treated at 500 °C has a low C/N ratio of 0.721; this means that the slightly decreased photocatalytic activity is attributed to the uncompleted condensation. However, there is a big drop in photocatalytic activity for the samples treated at temperatures above 520 °C. Apparently, increasing the heat temperature will decrease the photooxidation ability of g-C3N4. The C/N ratio, i.e., the degree of condensation, is 10400 DOI: 10.1021/la900923z

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inconsistent with the structural integrality. To obtain a sample with high C/N ratio, a high heat treatment temperature is needed to lower the content of hydrogen in products, which leads to decreased structural integrality for the final product. Indeed, in combination with the elemental analysis and thermal analysis results discussed above, it is obvious that in the case of g-C3N4 preparation by pyrolysis of melamine the degrees of condensation for the final product increase with increasing heat temperature, but the thermal stability of g-C3N4 decreases. This probably implies that, in our case of g-C3N4 preparation, 520 °C is an appropriate temperature for obtaining the g-C3N4 with good crystal structure and moderate degree of condensation, and therefore, the M520 sample exhibits high activity in degrading MO. Furthermore, it is worth pointing out that a one-time increase in the rate of MO photodegradation was achieved over the M520 sample under full arc light irradiation; the dye of MO can be degraded completely after 2.5 h UV-vis light irradiation. Some active species, such as the hydroxyl radicals ( 3 OH), the superoxide (O2 3 or HOO 3 ), and the holes, are formed during the photodegradation reaction induced by light irradiation. The 3 OH in aqueous solutions, as the primary oxidant, is generated via the direct hole oxidation19 or photogenerated electron induced multistep reduction of O2 (O2 þ e f O2 3 , O2 3 þ e þ 2Hþ f H2O2, H2O2 þ e f 3 OH þ OH-).20 Generally, it is confirmed that the generation of superoxide is associated with the photogenerated electron induced direct reduction of O2 (O2 þ e f O2 3 ). In addition, the photogenerated hole can directly react with organic compounds if the semiconductor photocatalyst has moderate redox potential. For these active species, hydroxyl radical reactions are nonselective and will virtually react with almost all the organic compounds by either H-atom abstraction, direct electron transfer, or insertion. In order to investigate the possible photodegradation mechanism of MO over g-C3N4 semiconductor, several comparison experiments were performed, and the results were shown in Figure 5b. Compared with photodegradation of the pure MO / g-C3N4 solution, we can know that no change in the rate of MO photodegradation is observed when methanol (10 vol %) was added as a sacrificial hole acceptor. This implies that the photogenerated hole is not an effective active species during degrading MO over g-C3N4; this reaction is not attributed to the direct hole oxidation. Indeed, as reported by Wang et al.,6 the oxidation level for water splitting is located slightly above the top of the valence band of g-C3N4, which would permit transfer of holes, but with a low driving force. This suggests that the low driving force is not beneficial for the hole reactions in aqueous solution system. However, there is a big increase in rate of MO photodegradation when Ag particles (5 wt %) were loaded onto the g-C3N4 surface using the photodeposition method;21 the MO can be degraded completely after 1 h visible light irradiation, which is 5 times faster than that over the bare g-C3N4. A further investigation, the effect of O2 on photodegradation of MO, was preformed in argon atmosphere. The reaction setup was vacuum-treated several times in order to eliminate O2, and then, high-purity argon gas (purity, 99.999%) was followed into the reaction setup for obtaining ambient pressure. After 5 h visible light irradiation, only 43% MO was photodegraded in the absence of O2 (see Figure S2 in Supporting Information). It means that O2 is a main factor for MO photodegradation over the g-C3N4 photocatalyst, which affects the formation of superoxide via direct reduction of O2 (19) Yoon, S. H.; Lee, J. Environ. Sci. Technol. 2005, 39, 9695–9701. (20) Liu, G. G.; Li, X. Z.; Zhao, J. C.; Horikoshi, S.; Hidaka, H. J. Mol. Catal. A: Chem. 2000, 153, 221–229. (21) Tada, H.; Ishida, T.; Takao, A.; Ito, S. Langmuir 2004, 20, 7898–7900.

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and hydroxyl radicals via multistep reduction of O2. A valuable fact is that, if the strong acid radical ion, such as SO42- (0.01 M) or NO3- (0.01 M), was introduced into the aqueous MO/g-C3N4 solutions, the MO degradation is significantly accelerated, and the reaction was completed after 80 min visible light irradiation. Clearly, the improved reaction activity is due to the introduction of a strong acid radical ion, which increases the Hþ concentration and therefore accelerates the reaction of O2 3 þ e þ 2Hþ f H2O2, further accelerating the formation of hydroxyl radicals via multistep reduction of O2. The experimental fact suggests that g-C3N4 photocatalyst possesses good environmental adaptability, which can directly apply to industrial wastewater treatment to degrade organic pollutants. On the basis of the aforementioned evidence, we are inclined to believe that for the MO photodegradation over g-C3N4 the photocatalytic activity has mainly resulted from active species which originated from photogenerated electron induced reduction of O2. For practical application of the photocatalyst, the stability during photoreaction was a crucial factor. Our stability test indicates that no decrease in photocatalytic activity was observed in the MO photodegradation reaction that was repeated three times (see Figure S5 in Supporting Information). The XRD pattern of the as-prepared g-C3N4 is similar to that of the sample after reaction (see Figure S6, in Supporting Information), meaning that the g-C3N4 is stable in the photoreaction. The apparent quantum efficiency is a criterion to evaluate photocatalytic activity of a given material. For MO degradation, taking into account a single electron process,22 the apparent quantum efficiency (denoted as EQ) for degrading MO was defined by EQ = Nnup/Nnip, where Nnup and Nnip represent the number of used photons and the number of incident photons, respectively. It is assumed that all incident photons are absorbed by the photocatalyst. The calculated value of EQ at 420 nm is (22) Bandara, J.; Morrison, C.; Kiwi, j.; Pulgarin, C.; Peringer, P. J. Photochem. Photobiol. A 1996, 99, 57–66.

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1.5%, implying that the low-cost g-C3N4 has potential in water purification due to its good photodegradation performance and environmental applicability.

Summary In summary, we have successfully fabricated the g-C3N4 photocatalyst by directly heating melamine. The stable MO dye was selected as a degrading goal to evaluate the photocatalytic activity of g-C3N4. Our results clearly indicate that the metal-free g-C3N4 has good performance in the photooxidation of organic pollutant. For the typical MO dye photodegradation, the photocatalytic avtivity of g-C3N4 can be improved significantly when Ag is used as a cocatalyst. This means that g-C3N4 is a promising material possessing a good potential in photocatalytic application fields if some techniques, such as loading cocatalyst and doping, are used to improve its photocatalytic activity. Our comparison studies showed that the photodegradation activity of MO over g-C3N4 is mainly attributed to the generation of active species induced by photogenerated electrons. Moreover, the g-C3N4 photocatalyst showed good photocatalytic activity in the presence of a strong acid radical ion; this further indicates that it is feasible to apply the g-C3N4 with low cost and facile synthesis to treat industrial wastewater containing organic pollutants. Acknowledgment. This work is supported by the National Natural Science Foundation of China (no. 20528302), the National Basic Research Program of China (973 program, 2007CB613301 and 2007CB613305). Supporting Information Available: UV-visible spectroscopic changes for MO degradation, XRD and SEM for Ag-modified g-C3N4, and the stability test for g-C3N4. This material is available free of charge via the Internet at http:// pubs.acs.org.

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